Placeholder negate method to get around first issue

All match data values now have a negate method, but it might not do 
anything.
With this change, a segfault occurs during output.

Invalid memory access (signal 11) at address 0x0
[0x55f0f6604906] *CallStack::print_backtrace:Int32 +118
[0x55f0f65f52c0] __crystal_sigfault_handler +192
[0x7fa1f7a8a890] ???
[0x55f0f6612cf6] *Slice(T):🆕read_only<Pointer(UInt8), Int32, 
Bool>:Slice(UInt8) +22
[0x55f0f6608265] *String#to_slice:Slice(UInt8) +37
[0x55f0f6608208] *String#to_s<String::Builder>:Nil +24
[0x55f0f6611b91] *String::Builder +17
[0x55f0f667ab7a] 
*Spectator::Matchers::EqualityMatcher::MatchData(String, String) +26
Program exited because of a segmentation fault (11)

src/spectator/expectations/expectation.cr

@@ -25,7 +25,7 @@ module Spectator::Expectations
         if @negated
           labeled_values.each do |labeled_value|
             value = labeled_value.value
-            value.negate if value.responds_to?(:negate)
+            value.negate
           end
         end
       end

src/spectator/matchers/match_data_value.cr

@@ -2,5 +2,9 @@ module Spectator::Matchers
   # Abstract base for all match data values.
   # All sub-classes are expected to implement their own `#to_s`.
   private abstract class MatchDataValue
+    # Placeholder negation method.
+    def negate
+      # ...
+    end
   end
 end